Oxygen vacancy defects (VO) in Ti-based oxides play important roles in catalytic processes despite limited knowledge regarding their formation and characterization. Here, we demonstrate the use of X-ray absorption spectroscopy (XAS) measurements to compare the relative proportion of VO defects in as-grown alkali hexatitanate A2Ti6O13 (A = Li, Na, K). Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions were studied. The similarity of measured XANES spectra of Ti K-edge in all samples indicates the presence of (Ti4+)O6 units in good agreement with reported X-ray diffraction results. The small influence of cations A at the tunnel was observed and can be well reproduced in the simulated spectra. In addition, we present a semi-quantitative approach to intuitively determine the content of VO defects in oxygen-deficient K2Ti6O13-x by in situ time-resolved XAS measurements under reducing conditions (10%H2/Ar, 50-650 °C). The in situ XANES measurements indicate that the oxidation state of bulk Ti remains the same as the as-grown sample, i.e., 4+, at elevated temperatures. By in situ EXAFS measurements, the relative number of VO defects is highest at a reduction temperature of ∼550 °C and slightly decreases after that. To confirm the formation of VO defects, first-principles calculations were independently carried out using a 126-atom K2Ti6O13 supercell with VO at various positions. Based on calculated EXAFS, the removal of the oxygen atom nearest to the tunnel, which is the lowest energy structure, provides a good match to the experimental spectra.
Most transparent conducting oxides (TCOs) exhibit n-type conductivity and are difficult to dope into p-type. Therefore, the development of efficient p-type TCOs is challenging. ZnRh2O4 spinel has been recognized as a potential p-type TCOs. However, the source of its p-type conductivity has not been elucidated. In this study, we used hybrid density functional calculations to investigate the energetics and electronic properties of native defects in ZnRh2O4, including vacancies, interstitials, and cation antisites. We found that all acceptor-type defects including Zn vacancies, Zn antisites, and Rh vacancies acted as deep centers. Charge neutrality analysis suggested that undoped ZnRh2O4 may behave as a p-type semiconductor with hole concentrations of 1018–1019 cm−3 under the extreme O-rich/Rh-poor growth condition in which ZnRh has a low formation energy and acts as the major source of hole carriers. However, under realistic growth conditions, the experimentally determined hole concentration significantly exceeds that which is calculated. Our results suggest that native point defects are unlikely to be responsible for the high hole concentrations observed in ZnRh2O4 spinel.
The highest theoretical Li storage capacity (2297 mA h g−1) to date in a pentagonal structure, achieved through aluminum-for-boron substitution in penta-BN2.
The elastic properties of the alkali hexatitanate family A2Ti6O13 (A = H, Li, Na, K, and Rb) are investigated which based on Density Functional Theory (DFT) within Generalized Gradient Approximation plus Hubbard U (GGA+U) approach. The results showed that all members of the family are wide-band semiconductors and the calculated lattice parameters are consistent with experimental values. In terms of mechanical stability, the results indicated that the alkali hexatitanates are highly incompressible to uniaxial stress, with the largest elastic constant C22 reaching values as high as 265 GPa in K2Ti6O13. The obtained elastic constants, using the stress-strain method, were used to calculate bulk modulus, shear modulus, Young's modulus, brittleness and ductility, elastic anisotropy, Vickers hardness, sound velocities, and the Debye temperature. It was found that the member of the family with the highest atomic number of the alkaline group, Rb2Ti6O13, had the highest values of bulk, shear, and Young's modulus, as well as the lowest values of shear and compression anisotropy, and a high Vickers hardness.
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